Site preparation and conditions

Site preparation

The location of the monitoring points was determined by York City Council, following discussion with the other relevant parties. The works necessary to provide access for HLE to install the monitoring equipment was undertaken by staff from YAT. In addition, YAT staff were present in a supervisory role during the installation process. Staff from the Environmental Archaeology Unit (EAU) at the University of York were also consulted during the installation process, and they assisted in the positioning of moisture cells.

Within the building, the preparatory work conducted by YAT involved excavating a 10.0m long and 1.0m wide trench to a depth of 1.4m. All depths are reported as metres below the upper surface of the former shop floor, which is referred to as ground level, and had been surveyed into Ordnance Datum by YAT. From Parliament Street to the rear of the building the trench ran in approximately a south-west to north-east direction. The trench bisected the original evaluation trench excavated by YAT in November 1994 (Brinklow 1995), which was reopened to create a 3.0 by 3.0m 'inspection chamber'. Once the monitoring equipment had been installed, backfilling of the trench and forming of the final inspection chamber was undertaken by YAT.

In the pavement of Parliament Street, three access points were created by YAT. For each point, one or two pavement slabs were lifted and the layer of reinforced concrete immediately below broken with a 'jack hammer'. A sufficient area was hand cleared to create a 750mm deep, and approximately 400 by 400mm diameter, pit. A full search of all buried services had been conducted prior to the lifting of the pavement slabs, and care was exercised at all time during the excavation work.

Following equipment installation, a manhole cover was placed over each access point and the pavement then reinstated by YAT. The manhole covers were supplied by HTS (formerly Hunting Land and Enviroment).

The monitoring points installed within the pavement area were surveyed into Ordnance Datum by HLE. For this purpose a Ordnance Survey benchmark search had been commissioned prior to the commencement of site work. The benchmark used by HLE is shown below, and full details of the search are given in Table 19.

Table 43: Ordnance Survey benchmark details

National Grid ReferenceDescription of markHeight above ground (m)Altitude (m AOD)Date verified
SE 6042 5174Rivet on All Saints Church, north ang.0.015.0411956

The location of the monitoring points and YAT excavated trench are shown in Figure 5.

Site conditions

The archaeological deposits into which the monitoring equipment has been installed does not fit into standard soil classification schemes, other than in the general category of 'made ground'. Made ground is not strictly soil because it is ground filled by human activity, rather than formed as a result of geomorphological processes. The monitoring technologies used in this project have been developed over many years for use in a wide variety of soil types, including those modified, but not totally formed, by human activity. The application of the monitoring technologies to made ground is relatively new and, unsurprisingly, there is a lack of published data to refer to, particularly with regard to calibration of the equipment.

The archaeological deposits have, however, been regarded as soil material, in so far as to subject them to a number of standard soil tests. The results of the analysis are important to characterise the deposits for descriptive purposes, assess if they will act in an aggressive manner to the buried equipment and to determine the reliability of monitoring data retrieved.

Material recovered from borehole 2 indicates that natural ground occurs at approximately 6.5m OD. Described as a slightly calcareous dark greyish-brown silty material, with an alkaline pH (8.2) and relatively low organic matter content (7.3%), it is possible that this is buried/relict soil.

Figure 12: Borehole 2 core samples
borehole 2 cores

Three of the cores from borehole 2 are shown in Figure 12. The left-hand core, from between 11.69 and 10.94m OD (1.80 to 2.55m bgl), shows a dark grey or black loamy 'peaty' deposit in which there are a few fragments of builders' debris and abundant wood remains. The middle core, from between 9.24 and 8.49m OD (4.25 to 5.00m bgl) contains a black loamy 'peaty' material with abundant wood remains. The right-hand core, from between 7.99 and 7.24m OD (5.50 to 6.25m bgl), contains a very dark grey/black organic silty/clay and a few limestone or masonry stones. In the figure, the builders' debris and stone shows up as the white areas, and the wood as the yellow areas. All depths are approximations.

Natural ground was not encountered in any of the other boreholes, possibly because they were shallower, terminating at between and 9.43 and 7.35m OD. Those boreholes that passed to at least 8.5m AOD did reveal a stiff non-calcareous very dark greyish-brown clay. However the presence of charcoal flecks within this material indicates that, though a 'soil', it had been modified by human activity.

The deposits overlying the dark greyish-brown material are variable and represent the occupational layers of York city centre. The matrix of the deposits generally had the following characteristics: dark brown or black colour, low bulk density (less than 1.1 g/cm ) and high organic matter content (greater than 12%). Contained within this matrix were quantities of domestic and industrial waste, and builders' debris. Characterisation of the site has been attempted by reference to a proposed urban soil classification scheme. Assuming that the natural described from borehole 2 is a buried soil, the York deposits conform to a 'compost-deepened' soil (Hollis 1992). This term applies to all deepened soils with a surface horizon more than 400mm thick, resulting from the addition of earth-containing manures or waste materials from former human occupation (middens). In addition, because of the generally high organic matter content, the deposits can more correctly be referred to as a compost-deepened organic soil. This soil description will be assumed when interpreting the monitoring data.

An assessment of the deposits' potential aggressiveness to the installed monitoring equipment is made from electrical conductivity (EC) measurements, as these are indicators of the total quantities of soluble salts in a sample. Compared to a classification scheme used for agricultural soils, the EC values of the deposits indicate saline conditions of varying severity. The interpretation used is that an EC value of between 4 and 8 ms/cm is typical of slightly saline conditions, values between 8 and 15 ms/cm are moderately saline, and above 15 ms/cm is strongly saline (Landon 1984). An EC value below 4 ms/cm is indicative of a non-saline soil. The deposits have EC values of between 2.43 and 17.78 ms/cm.

It is important that a classification of deposits also make reference to EC measurements of groundwater (or soil water) samples. This is because the deposit analysis reveals a total potential reservoir of salts but does not take into account their solubility. Though all naturally occurring water contains some amount of dissolved salts, the five water samples analysed had relatively high EC values of between 5.11 and 5.95 ms/cm. This indicates that a proportion of the salts present are in a soluble form. In conclusion, the deposit's salt content has the potential to act in an aggressive manner to buried metal objects, and will also influence the moisture cell data.

Analysed samples from upper sections of the archaeological deposit report higher EC values than from the base of the sampled profile. For example, the EC of samples from the top and bottom of borehole 4 were 4.99 and 2.59 ms/cm respectively. The top sample was taken from the deposit at approximately 1.85m below ground level. Samples recovered during installation of the moisture cells were also analysed for EC and again it appeared to decrease with depth. For example, the EC of samples from 0.98, 1.20 and 1.80m down the south-facing section of the deposit were 13.94, 13.32 and 9.08 ms/cm respectively. This characteristic of decreasing EC with depth was most evident in deposits sampled from within the building, rather than those below the pavement slab at borehole 2 location. A redundant concrete floor lay directly over the deposits, and it is possible that soluble salts from the concrete may have migrated down into the deposits where they then precipitated out. Possible visible evidence of this were accumulations of a white powdery substance in the deposit. This has not as yet been formally identified but could possibly be Calcium humate, which is a fulvic acid salt formed by reactions between organic acids (formed by the decay of organic matter) and concrete.

In addition to EC measurements, a determination of individual salts is used to assess corrosion risks, in particular sulphate attack. The most abundant salts are generally calcium sulphate, magnesium sulphate and sodium sulphate. The latter two salts are more soluble than calcium sulphate, which possibly explains why calcium levels within the deposits were higher than magnesium and sodium levels and the reverse was true for the water samples; see Table 44.

Table 44: Sample analysis from borehole 2

ParameterDeposit sample from near base of borehole 2Water sample from base of borehole 2
Calcium31000 mg/kg108 mg/l
Magnesium5240 mg/kg380 mg/l
Sodium884 mg/kg276 mg/l

Sulphate attack is normally associated with the deterioration of building materials, particularly concrete and cast iron piping. The monitoring equipment most likely to be affected by high sulphate concentrations are the moisture cells and suction samplers (because of their reliance on a porous ceramic cup).

Sulphates are a natural constituent of uncontaminated soils, and typical values range from 0 to 2000 mg/kg (Anon 1987). The archaeological deposits contained sulphate levels of between 776 and 8250 mg/kg, with the highest values recorded from the three near surface samples. Based on Building Research Guidelines (BRE), the reported sulphate levels in both deposit and water samples were in Class 1, except for the near surface samples that had levels in Class 2 (Anon 1991). The BRE guidelines recognise five soil classes: soils within Class 1 are considered non-aggressive, and within Class 2 they impose a slight risk of sulphate attack on concrete. Therefore the deposits do not pose a significant hazard to the water sampler's ceramic cup. Within the surface of the deposit, there is, however, some risk of metal corrosion if the level of soluble sulphate rises above 1000 mg/kg. It was also noted that the surface deposits had a lower pH than those samples from the base of the profile, indicating more acid and so more aggressive conditions.

Also determined were the deposit's sulphide content which, except for one sample, were at or below 30 mg/kg. Because the deposits contain appreciable concentrations of sulphate from which sulphides are formed (by sulphate reducing bacteria), and a natural soil typically has a sulphide content of between 0 and 10 mg/kg, the values for the York site are considered low. It is possible that an under-estimation has occurred due to the unavoidable exposure of deposits to the atmosphere during sampling collection, and the time taken for laboratory analysis of the samples. On exposure to oxygen, sulphides can form hydrogen sulphide gas which has a characteristic 'rotten-egg' odour, and this was detected during construction of all eight boreholes. Because hydrogen sulphide is highly toxic, all the site work was undertaken in a well-ventilated environment.

The sulphate reducing bacteria (e.g. Desulfovibrio) are one of many types that exist in a soil environment. A bacteriological examination of two water samples from the dipwell show that active aerobic and anaerobic bacterial populations are present at the project site. Though a careful sampling procedure was followed, it is probable that the aerobic count is higher than actual because of unavoidable contamination of the sample. The examination therefore indicates that conditions are anaerobic (waterlogged and low oxygen levels) because of the relatively small difference between the aerobic and anaerobic counts. Had aerobic conditions been present, the high organic matter content of the deposits would have supported larger populations of aerobic bacteria and, due to competition, a lower anaerobic bacterial population.

Finally, the results of a metal determination from four of the borehole samples were compared to a set of guidelines for the classification of contaminated soils (Anon 1980). For each parameter analysed the level in the deposit was close to, or within, that typically found in an uncontaminated soil. The only exception was magnesium, with high values recorded. A similar determination conducted on the water samples also reports low metal contents, with no demonstrable pollution indicators. The site appears to contain no metal pollutants that would pose a hazard to the installed monitoring equipment.


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Last updated: Thur Feb 28 2002